Fix platform tag logic to generate the same as cpython on AIX

[pgimalac]

Fix the platform tag logic on AIX to generate the same tag as CPython does.

On AIX, CPython runs lslpp -Lqc bos.rte and parses the output to extract version information, then builds the tag with format aix_{ver:x}{rel}{tl:02}_{builddate:04}_{bitsize}.
See https://github.com/python/cpython/blob/main/Lib/_aix_support.py for the exact implementation.

maturin currently falls through the POSIX fallback which uses uname() 's release and machine fields, producing tags like aix_3_00F9D80F4C00 (hardware model number) that pip does not recognize.

This PR aims to implement the same logic so that wheels are properly tagged and can be used by pip.
Without the fix, pip install fails with ERROR: not a supported wheel on this platform.

Added unit tests to validate the parsing.
Also built maturin with the fix, generated a small test wheel, installed it with pip, and called it to validate it all works end to end on AIX.

[Copilot]

Pull request overview

This PR updates maturin’s wheel platform tag generation on AIX to match CPython’s sysconfig.get_platform() behavior, fixing tags that pip previously rejected.

Changes:

• Add an AIX-specific platform tag path using lslpp -Lqc bos.rte parsing to produce CPython-compatible aix_{ver}{rel}{tl}_{builddate}_{bitsize} tags.
• Introduce a unit-testable parser helper (aix_tag_from_lslpp_output) and add unit tests covering typical and edge-case outputs.

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